Most modern X-ray hospital equipment not only records images on radiographic plates but also can produce a secondary light image on a scintillation screen which may be viewed directly, photographed with a still or moving picture camera or, as is relevant to the present invention, the secondary light image may be viewed with an electro-optical scanner device which converts the secondary image into corresponding electrical signals for display on a cathode ray tube, for example. The term electro-optical device includes inter alia mechanical photoelectric facsimile scanners, television camera tubes of all types, image intensifiers and similar devices, which may have a bias or control electrode by which the gain of the device and its output signal amplitude may be varied, as well as the lenses, mirrors and similar optics associated with the device, which can also be used to vary the light intensity and thus the overall gain.
Image tubes or flying spot scanners rely on the photoelectric effect, whereas a scintillation screen produces its secondary light image by 1 minescence. Consequently the dynamic range of the scanners and screen differ considerably. Dynamic range is the ratio of lhe brightest to the least bright light point of an image source such as the scintillation screen, or the ratio of brightest to least bright light intensity to which an electro-optical scanner can respond. The dynamic range of the screen will be influenced by the type of X-ray procedure, the X-ray tube voltage, the dimensions and proportions of bone, tissue and air of the subject. The light from a scintillation screen, taking into account the loss in the lens system projecting the image on the scanner, can vary throughout the image with a dynamic range of 1000. A typical television camera tube for such low light levels has a linear response over a dynamic range of only 100 to 150. Thus a typical camera tube can respond linearly to only a fraction, e.g., one tenth, of the dynamic light range of an available scintillation screen. Obviously detail, resolution and contrast are lost in converting the scintillation image to electrical signals.
Additionally the optical inefficiency of the lens system projecting the scintillation image on the scanner reduces light received by the tube not only generally but particularly by operation of the cosine law which reduces light from the corners of the scintillation screen image most remote from its center. The particular light reduction distorts and reduces contrast in the portions of the electrical signal and display corresponding to the corners of the scintillation image.
Electro-optical apparatus which reproduces substantially the full dynamic range of light intensity values throughout an image area is disclosed in copending application Ser. No. 457,678 entitled MULTIPLE X-RAY IMAGE SCANNERS, and filed Jan. 13, 1983, now U.S. Pat. No. 4,504,859. Therein is disclosed X-ray apparatus with a scintillation screen showing a light image in an area viewed by two or more video camera tubes. The tubes receive a projection of the light image in two different ranges of light intensities and generate electrical signals corresponding to the light intensities. The signals are combined to reconstruct the image.
It is the object of the present invention to provide an improved way to match the sensitivities of the camera tubes to the different ranges of the light image.